Not willing to work with other people, or not willing to help when someone asks. It describes a person, or sometimes a group, who will not join in — they refuse, they answer as little as possible, they let you do the work alone. Two flavours worth separating. Sometimes it means actively refusing: the witness was uncooperative, so the police learned nothing. Sometimes it just means unhelpful in a passive way: an uncooperative student who does not share materials in group work. Either way, the person could help and chooses not to. The word is fairly formal and a bit cold. You will meet it in reports, in school comments, in news about investigations and negotiations — 'the government remained uncooperative'. Face to face, people usually say something warmer or shorter: 'he wouldn't help', 'she's being difficult'. Because it is a judgement written by someone in authority, it often carries a hint of blame: the person writing it feels they were owed cooperation. One playful stretch: English speakers use it about objects and bodies that will not do what they should. 'My hair is being uncooperative today.' 'The printer is uncooperative.' The thing has no will, of course — that is the joke.